As a hybrid driving force transmission device, there has heretofore been known one in which an engine, a motor clutch unit and a transmission unit are coupled, as disclosed in Japanese Patent Application Publication No. 2006-137406. Among these, the motor clutch unit includes a clutch hub coupled to an output shaft of the engine, a clutch cover having a rotor of an electric motor fixed thereto, and coupled to a transmission input shaft, a multi-plate dry clutch interposed between the clutch hub and the clutch cover, and a slave cylinder (generally called a “CSC.” Note that “CSC” is an abbreviation for Concentric Slave Cylinder) for hydraulically controlling the engaging and disengaging of the multi-plate dry clutch.